This invention relates to the synthesis of hydrocarbons from a methane source. A particular application of this invention is a method for converting natural gas to more readily transportable material.
A major source of methane is natural gas. Other sources of methane have been considered for fuel supply, e.g., the methane present in coal deposits or formed during mining operations. Relatively small amounts of methane are also produced in various petroleum processes.
The composition of natural gas at the wellhead varies but the major hydrocarbon present is methane. For example, the methane content of natural gas may vary within the range from about 40 to about 95 volume percent. Other constituents of natural gas include ethane, propane, butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen.
Natural gas is classified as dry or wet depending upon the amount of condensable hydrocarbons contained in it. Condensable hydrocarbons generally comprise C.sub.3 +hydrocarbons although some ethane may be included. Gas conditioning is required to alter the composition of wellhead gas, processing facilities usually being located in or near the production fields. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
Large scale use of natural gas often requires a sophisticated and extensive pipeline system. Liquefaction has also been employed as a transportation means, but processes for liquefying, transporting, and revaporizing natural gas are complex, energy-intensive and require extensive safety precautions. Transport of natural gas has been a continuing problem in the exploitation of natural gas resources. It would be extremely valuable to be able to convert methane (e.g., natural gas) to more readily handleable or transportable products. Moreover, direct conversion to olefins such as ethylene or propylene would be extremely valuable to the chemical industry.
U.S. Pat. No. 4,199,533 discloses a process for converting methane to higher molecular weight hydrocarbons by using chlorine gas as a recyclable catalyst. The process produces ethylene as a major product along with hydrogen chloride, which is converted to chlorine for recycle in the system. Major drawbacks of the '533 process are the large amount of chlorine required, the necessity of regenerating chlorine from hydrogen chloride to maintain an economically viable system, and the need to use operating temperatures in excess of 1000.degree. C. to produce ethylene.
Recently, it has been discovered that methane may be converted to higher hydrocarbons (e.g., ethane, ethylene and higher homologs) with minimal formation of carbon oxides by contacting methane with a reducible metal oxide as a selective oxygen source. As the methane is converted to hydrocarbon products and coproduct water, the active oxygen of the metal oxide is depleted, resulting in a reduced metal oxide. The reduced metal oxide is relatively inactive for the oxidative conversion of methane but active oxygen may be replaced by regeneration of a reducible metal oxide. Such regeneration is accomplished by reoxidation of the reduced metal oxide.
Reducible oxides of several metals have been identified which are capable of converting methane to higher hydrocarbons. Oxides of manganese, tin, indium, germanium, lead, antimony and bismuth are particularly useful. See commonly-assigned U.S. patent application Ser. Nos. 522,925 now U.S. Pat. No. 4,443,649; 522,944 now U.S. Pat. No. 4,444,984; 522,942 now U.S. Pat. No. 4,443,648; 522,905 now U.S. Pat. No. 4,443,645; 522,877 now U.S. Pat. No. 4,443,647; 522,876 now U.S. Pat. No. 4,443,644; and 522,906 now U.S. Pat. No. 4,443,646; the entire contents of which are incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 522,935 discloses and claims a process which comprises contacting methane with an oxidative synthesizing agent under elevated pressure (e.g., 2-100 atmospheres) to produce greater amounts of C.sub.3 +hydrocarbon products. The entire content of this application is incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 522,938 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with particles comprising an oxidative synthesizing agent which particles continuously recirculate between two physically separate zones--a methane contact zone and an oxygen contact zone. The entire content of this application is incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 522,937 disclosed and claims a process for the conversion of methane to higher hydrocarbon which comprises contacting methane with an oxidative synthesizing agent containing a promoting amount of alkali metal and/or compounds thereof. The entire content of this application is incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 522,936 filed Aug. 12, 1983 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with an oxidative synthesizing agent containing a promoting amount of alkaline earth metal and/or compounds thereof. The entire content of this application is incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 06/600,665 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with a contact solid comprising a reducible oxide of praseodymium and at least one member of the group consisting of alkali metals, alkaline earth metals and compounds thereof. The entire content of this application is incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 06/600,918 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with a contact solid comprising a reducible oxide of terbium and at least one member of the group consisting of alkali metals, alkaline earth metals and compounds thereof. The entire content of this application is incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 06/600,917 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with a contact solid comprising a reducible oxide of cerium and at least one member of the group consisting of alkali metals, alkaline earth metals and compounds thereof. The entire content of this application is incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 06/600,730 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with a contact solid comprising a reducible oxide of iron and at least one member of the group consisting of alkali metals, alkaline earth metals and compounds thereof. The entire content of this application is incorporated herein by reference.
Commonly-assigned U.S. patent application Ser. No. 06/600,969 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with a contact solid comprising a reducible oxide of ruthenium and at least one member of the group consisting of alkali metals, alkaline earth metals and compounds thereof. The entire content of this application is incorporated herein by reference.
As noted, the reaction products of such processes are mainly ethylene, ethane and other light hydrocarbons, carbon oxides, coke and water. It would be beneficial in these processes to reduce selectivities to carbon oxides and coke and to increase methane conversions to the desired hydrocarbon products.
Accordingly, an object of this invention is to provide an improved process for converting methane to higher hydrocarbons wherein methane is contacted with a reducible metal oxide as a selective oxygen source. A further object of this invention is an improved selective oxygen source--a contact solid capable of enhancing the selective conversion of methane to higher hydrocarbon products. Other aspects, objects and the several advantages of this invention will become apparent to those skilled in the art upon reading this disclosure and the appended claims.